1. Field of the Invention
This invention relates to electro-optic modulators, particularly to electro-optic modulators utilizing electro-optic crystals.
2. Description of the Prior Art
Crystalline electro-optic modulators generally exhibit some optical absorption at all wavelengths. When a beam of light of sufficiently high average power passes through the modulator, significant heating of the crystal can result. The associated thermal distribution in the crystal produces strain-induced birefringence; the direction of the resultant strain axes and their birefringent effect on the phase of a transmitted optical beam is dependent on the crystal structure and the exact details of the intra-crystal temperature profile. In general, these thermally induced axes are physically oriented such that any phase retardation impressed on a linearly polarized input beam is a function of position within the beam cross-section. The non-uniform phase retardation can severely limit the modulation depth or extinction ratio for high power optical beams. In applications where the modulator is placed within a laser resonant cavity, the non-uniform phase retardation can severely limit the attainable extinction ratio from the laser and can produce exceptionally high losses if passed through a polarizer.
One example of the prior art is disclosed in U.S. Pat. No. 3.900,247 issued to S. G. Zaky on Aug. 19, 1975, entitled "Optical Modulator Having Compensation for Thermal and Space Charge Effects." Zaky shows optical radiation passing through an electro-optic crystal through a quarter wave plate to a mirror whereupon the radiation is reflected back through the quarter wave plate and through the electro-optic crystal. The mirror is spaced apart from the electro-optic crystal to provide a transit time to allow the modulated birefringence of the electro-optic crystal to be changed before the radiation passes back through the crystal. The 180.degree. relative phase shift of the radiation due to the quarter wave plate and the mirror nullify any steady birefringence caused by thermal or space charge effects in the electro-optic crystal. Modulation of the radiation is obtained by changing the birefringence of the modulation element during the finite transit time of the modulator.
In view of the prior art, it is therefore desirable to thermally compensate an electro-optic modulator independent of transit time of the radiation through the modulator. It is desirable to thermally compensate electro-optic modulators wherein the radiation transmitted therethrough is of sufficient power to cause localized heating in the light transmitting materials of the modulator. In place of total compensation, it is desirable to provide partial compensation of the thermal birefringence in the electro-optic modulator to provide an improvement in the modulator performance. It is desirable that the thermally compensated electro-optic modulator is physically small. It is further desirable that the thermally compensated electro-optic modulator be useable within a laser resonant cavity.